In optical fiber transmission systems, polarization multiplexing for performing signal multiplexing by utilizing polarization has been put into practical use. The use of polarization multiplexing can double the transmission capacity per fiber. Digital signal processing techniques have been recently adopted for reception units of optical transceivers, which makes it possible to effectively split polarization multiplexed optical signals.
In order to meet an increasing demand for communication traffic in the future, it is important to increase the transmission capacity by multiplexing polarization multiplexed optical signals at narrow wavelength intervals by dense wavelength division multiplexing. A wavelength multiplex transmission system that multiplexes and transmits a plurality of optical signals having different wavelengths within one optical fiber has a problem of cross-phase modulation (XPM) which is a non-linear effect in optical fibers. In the cross-phase modulation, an optical signal having a certain wavelength affects an optical signal having another wavelength, which results in deterioration of the transmission signal quality. In order to achieve an excellent transmission signal quality in the wavelength multiplex transmission system, it is important to reduce the influence of the cross-phase modulation.
For example, Patent Literature 1 discloses an optical wavelength multiplex transmission system capable of reducing waveform distortions due to the effect of cross-phase modulation in the optical wavelength multiplex transmission system employing an intensity modulation/direct detection (IM/DD) scheme. In this optical wavelength multiplex transmission system, different wavelengths of optical signals that are modulated by NRZ (Non Return to Zero) code are set in such a manner that bit patterns delivered from a plurality of optical transmitters that deliver the optical signals cause walkoff (difference in propagation distance) of ½ bits or more within the effective length of an optical fiber.
On the other hand, Patent Literature 2 discloses a polarization fluctuation compensation device that compensates for the effect due to cross-phase modulation to achieve an excellent reception characteristic. Patent Literature 3 discloses an optical field receiver suitable for receiving optical multilevel information transmitted through an optical fiber.